1. Field of The Invention
This invention is in the general field of radio transmitter power amplifiers for satellite communications, and more particularly in the field of voice and communications power amplifiers which simultaneously amplify for transmission two different signals on different carrier frequencies for communication via very small aperture terminals.
2. Description of the Related Art
It is often necessary to transmit two different data signals by means of a single transmitter. For example, an earth station for transmitting information from a remote computer terminal to a central computer via satellite must at various times transmit voice and computer data signals. Each data signal must be transmitted on a different carrier frequency, and a power amplifier for use in such a transmitter must be capable of amplifying a carrier signal of either frequency to a sufficient level of power for transmission.
One application for transmitters of this type is satellite communication terminals in which concurrent communications of voice and data is desired. It is highly desireable that the transmitter be co-located with each antenna at a user site to reduce the overall cost of such systems. However, such use places severe demands on the complexity, cost and performance of such systems.
For instance, if both voice and computer data signals are ready for transmission at the same time, the power amplifier must simultaneously amplify a carrier signal carrying the computer data signal and a carrier signal carrying the voice signal to a sufficient level of power for transmission. If the required power level is the same for each carrier signal, the amplifier must be able to provide a total power output of twice the maximum power which would have been required to transmit either carrier signal alone, a difference of three dB.
Moreover, when two carrier signals are being amplified simultaneously it is necessary to operate the amplifier some three to six dB below its maximum power level in order to avoid unacceptable intermodulation distortion between the two signals. Accordingly, a power amplifier which can simulataneously amplify two different carrier signals to a required level of power must have a maximum power rating six to nine dB higher than would be required to amplify only one signal to the same power level.
The power amplifier represents a significant part of the cost of a transmitter and, in a relatively small installation such as an earth station for a remote computer terminal, may represent a major fraction of the cost of the entire station. If it were possible to transmit both voice and computer data carrier signals by means of a power amplifier no more powerful than would be required to transmit only one signal, significant cost savings and increased efficiency of operation could be achieved.
Accordingly, there is a need for a way to transmit two different carrier signals, each carrying different data signals such as voice and computer data, without using a power amplifier more powerful than would be needed to transmit one such signal by itself.